1. Field of the Technique
The present invention relates to a supporting mechanism for a fixed detection section in a detection device, which comprises a rotary detection section attached to a hollow shaft fixedly connected through its hollow to one end portion of a rotary shaft of a drive motor the load of which is coupled to the other end portion opposite to the one end portion, and the fixed detection section supported by a fixed housing at a side opposite to the load of the drive motor and mounted through a bearing on the hollow shaft.
2. Prior Art
Conventionally, a detection device such as an optical encoder as a detector for a drive motor is mounted on one end portion of a rotary shaft of the drive motor the load of which is coupled to the other end portion opposite to the one end portion. The optical encoder has been coupled heretofore to the rotary shaft by means of an indirect coupling or a direct coupling, the former using, for example, gears and the latter coupling the shaft of the encoder to the shaft of the drive motor with a coupling.
The indirect coupling with gears or the like, however, has been found not satisfactory in that there is a transmission error due to the backlash of the gears and the mechanical parts used for the coupling occupy a large space in the device.
In the direct method using a coupling, although there occurs no transmission error, the structure of the device becomes longer in the axial direction, and the adjustment for alignment of the axes has been cumbersome.
Thus, in this field of technology, a method has been adopted in which the shaft of an optical encoder is made hollow and the encoder is directly coupled through its hollow to and about the rotary shaft of a drive motor.
More in particular, a fixed detection section in a detection device is constructed such that two bearings mounted on a hollow shaft fixedly connected to the rotary shaft of a drive motor support the fixed detection section. The fixed detection section must meet the following two requirements.
(1) Regardless of whether the drive motor is under rotation or not, the fixed detection section must not displace in an axial direction of the rotary shaft.
(2) In the case that the fixedly connected hollow shaft displaces by the same amount of displacement of the rotary shaft of the drive motor, an overload must not be applied to the bearings because the possible displacement of the fixed detection section might occur.
Upon consideration of these points, the present inventor has proposed a means for supporting a fixed detection section at a side of a fixed housing of a drive motor with the help of a plate spring, as shown in FIG. 1.
In the figure, reference numeral 1 designates a frame, 2 designates a printed circuit board for a detector device, 3 designates a sensor for use in the detector device, such as a photodiode, 4 designates a fixed detection section, 5 designates a plate spring, 6 designates a rotary disk, 7 designates a hollow shaft, 8 designates a bearing, 9 designates the rotary shaft of a drive motor, and 10 designates a fixed housing. FIG. 2 shows a perspective view of the plate spring 5, wherein opposite end portions of the plate spring 5 are formed with apertures 11 for use in coupling the fixed detection section 4 to the fixed housing 10 with screws. Thus, the fixed detection section 4 is supported through bearings 8 by the hollow shaft 7 and is also held through the plate spring 5 at the fixed housing 10.
With this plate spring 5, however, only a localized fixation at a point on a circumference on the fixed detection section 4 can be obtained. Therefore, there is a problem that a force is locally applied to the bearings 8 on the hollow shaft 7 as the displacements of the shafts occur.
In order to eliminate this disadvantage, another spring plate method may be thought of in which as shown in FIG. 3, a thin circular plate is used for the spring plate so as to make applied forces uniform throughout the circumference. The thin circular plate is generally a circular plate made of metal spring material having its thickness in the order of 0.05 to 0.3 mm. In the figure, 14 designates a hollow spacer, 15 designates a screw, and 13 designates an aperture formed in the thin circular plate spring 12 through which the hollow spacer 13 is to be inserted. FIG. 4 is a side elevational section in which the plate spring 12 of FIG. 3 is installed to the detection device. It is seen from FIG. 4 that the plate spring 12 is coupled, by threading the screws 15, to the fixed housing 10 of a drive motor and the fixed detection section 4, being spaced therefrom by the distance equal to the length of the spacer 14.
For the output signals from an optical encoder, it is frequently required to make the phases coincide with each other between the pulse generation position of the encoder and the certain position of the rotary shaft. In this case, the encoder is of the type that a particular position on the circumference can be identified, such as an origin point type encoder, an absolute type encoder, or the like. And the certain position of the rotary shaft of a drive motor corresponds to, for example, a key groove, or a particular magnet for a rotary magnetic field type alternating current servo motor. Here, two methods can be applied:
(1) adjusting when the hollow shaft of the encoder is coupled to the rotary shaft of the drive motor, or
(2) adjusting when the fixed detection section at the fixed side of the encoder is coupled to the fixed housing of the drive motor.
With the method (1), it is impossible to adjust while the drive motor is rotating. Therefore, it takes a long time to adjust, and it is difficult to adjust the phases with good precision. In general, the method (2) has been employed. However, with the above mentioned thin circular plate spring, it is impossible to adjust since the relative coupling position between the fixed detection section and the fixed housing is determined by the positions of the apertures formed in the circular plate spring.
Here, a further method may be thought of which enables the adjustment by changing opposing two apertures 13 of the circular plate spring 12 of FIG. 3 into elongated apertures which extend along the circumference. However, in this case also, there are problems that the distance along the circumference between the positions of the screws 15 at the fixed housing side and at the fixed detection section side become unequal, and that the elasticity characteristic varies with the relative coupling position.